A week of spaceflight tests, large and small, reminds us why NASA's much maligned launch vehicle is important. If it fails, we lose the moon.

This week, all eyes were on NASA as it conducted the first flight of the Ares I, the first launch vehicle the agency designed since the Space Shuttle. October also witnessed progress in other space launches, some of which are seen as possible replacements for the towering NASA rocket. But a comparison of the milestones met in the last two weeks shows why NASA's Constellation program remains important, and is exceedingly difficult to replace.

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Today's launch generated data to optimize the Ares I, which is being built to bring people into orbit. The Constellation plan is meant to replace the Shuttle (a space plane) with this large, Apollo style rocket and capsule combination. There is a chorus of complaints about the Ares I, stemming from its inability to be ready in time to service the International Space Station, its cost overruns, it's oversized capsule and delays from a vibration problem that required a shock-absorbing redesign.

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The critics are not wrong to attack Ares and its capsule, Orion. But here is a sobering fact: Without this NASA program, America will have no way to deliver astronauts into space without renting a Russian Soyuz spacecraft. The Shuttle is set to retire in 2010 after a long, hard service life. The system is aged and it will be expensive and probably dangerous to extend its life. Even if NASA can keep Ares and Orion on track, there will still be at least a six year gap between the first Ares I flight and the Shuttle's retirement. Chronic underfunding from the Bush and Obama administrations have hampered the program and belied both politician's expansive, pro-NASA prose. The money never matched the rhetoric.

There are alternatives to Ares, but none of them are going to bring people into space. A much-touted plan to use military launch vehicles, used to deliver satellites into orbit, seems like a good idea. But converting a cargo-carrying rocket to carry people--called "man-rating"--is more difficult than it sounds, especially when it's done to NASA's stringent safety standards. All components must be tested and retested, while redundant parts must be built and integrated onto the rocket. This will take time and money and will only get the United States a rocket used to place capsules into orbit. Ares is supposed to be the technical foundation for a larger rocket called Ares V that would bring hardware and supplies that could be used to go to the Moon and Mars. The Ares I would launch the astronauts and the capsule, while the larger rocket brings a lander and supplies for longer trips around our solar system. Without Ares I, expect more delays on our return to the moon, as China, India and Japan set their eyes on it as a destination for manned trips.

But what about private space? NASA wisely has invested billions of dollars in private space companies, hiring them to create rockets that can deliver cargo to the ISS. One of these, Space Exploration Technologies, on October 16th successfully test-fired a cluster of nine engines that will launch its Falcon 9 rocket. The test was the last needed validation before the engine is shipped to Cape Canaveral and placed on a rocket, which should happen next month. Company officials say the Falcon 9's demonstration flight will take place within three months after the launch vehicle arrives at Cape Canaveral. NASA desperately needs these private space companies to succeed, but these are not rated to bring people into orbit. And that again leaves us with foreign rentals to gain access to the ISS, and no hope to get a manned mission to the moon or beyond. Private space companies are nothing if not ambitious. They are interested in traveling to the moon--in theory--but are more obsessed with suborbital tourist trips or low earth orbit satellite deliveries. If you ask them about competition from NASA's moonshot, they know their limits. It takes prize money to get them to think about the moon. On Tuesday and Wednesday, as NASA launched Ares, another launch was conducted in the desert in Mojave, Calif.

Masten Space Systems became the last of three private space companies to design a lunar lander that can take off, hover at an altitude, transition to another location and then settle back down to land. The Northrop Grumman Lunar Lander Challenge is funded by NASA and presided over by the X PRIZE Foundation. "It would be good for everyone to take a look at what the private guys are doing," says Doug Graham, a spokesperson for Masten. "With the prize concept, the American taxpayers as a whole can get a lot more innovation for little money. You tend to have more innovation when you're not answerable to a large bureaucracy."

That may be true, but when you compare the humble X-prize craft--tethered and hovering after months of effort--to the enormous Ares I-X, the scope of a manned mission to another planet becomes a little more clear. NASA is certainly a massive, clumsy bureaucracy administered by a fickle, inconsistent government. It is also the best chance America has to land humans on Mars, to explore the limits of space-based medicine and manufacturing and perhaps even form a colony on the moon. Reform NASA, streamline its processes and make it more flexible and responsive--but we hope the Obama administration is not quick starve NASA of its sense of purpose, which the Constellation program represents if nothing else.